The 15 one-mark questions from the March 2026 12th Standard Physics public exam, in paper order, with the correct option marked and a short explanation.
Q1
Two identical conducting balls having positive charges \(q_1\) and \(q_2\) are separated by a centre to centre distance r. If they are made to touch each other and then separated to the same distance, the force between them will be :
- A. zero
- B. less than before
- C. more than beforeCorrect
- D. same as before
Explanation. After touching, each ball carries \(\frac{q_1+q_2}{2}\). Since \(\left(\frac{q_1+q_2}{2}\right)^2 \ge q_1q_2\), the force (proportional to the product of charges) becomes more than before (equal only if \(q_1=q_2\)).
Q2
Two wires of A and B with circular cross section are made up of the same material with equal lengths. Suppose \(R_A = 3R_B\), then what is the ratio of radius of wire A to that of B ?
- A. \(\frac{1}{\sqrt{3}}\)Correct
- B. 3
- C. \(\frac{1}{3}\)
- D. \(\sqrt{3}\)
Explanation. \(R=\frac{\rho L}{\pi r^2}\), so with same \(\rho\) and L, \(R\propto\frac{1}{r^2}\). \(\frac{r_A^2}{r_B^2}=\frac{R_B}{R_A}=\frac{1}{3}\), hence \(\frac{r_A}{r_B}=\frac{1}{\sqrt{3}}\).
Q3
A non conducting charged ring carrying a charge of q, mass m and radius r is rotated about its axis with constant angular speed \(\omega\). The ratio of its magnetic moment with angular momentum is :
- A. \(\frac{2q}{m}\)
- B. \(\frac{q}{2m}\)Correct
- C. \(\frac{q}{4m}\)
- D. \(\frac{q}{m}\)
Explanation. Current \(I=\frac{q\omega}{2\pi}\), so \(\mu=I\pi r^2=\frac{q\omega r^2}{2}\). Angular momentum \(L=mr^2\omega\). Hence \(\frac{\mu}{L}=\frac{q}{2m}\).
Q4
When the current changes from +2A to \(-\)2A in 0.05 s, an emf of 8 V is induced in a coil. The coefficient of self induction of the coil is :
- A. 0.4 H
- B. 0.2 H
- C. 0.8 H
- D. 0.1 HCorrect
Explanation. \(\varepsilon=L\frac{\Delta I}{\Delta t}\), with \(\Delta I=4\) A: \(L=\frac{8\times0.05}{4}=0.1\) H.
Q5
Which one of them is used to produce a propagating electromagnetic wave ?
- A. a charge moving with constant velocity
- B. an accelerating chargeCorrect
- C. a stationary charge particle
- D. an uncharged particle
Explanation. A stationary charge produces only an electric field and a uniformly moving charge produces steady fields; only an accelerating charge radiates electromagnetic waves.
Q6
The speed of light in an isotropic medium depends on :
- A. the nature of propagation
- B. its intensity
- C. the motion of the source with respect to medium
- D. its wavelengthCorrect
Explanation. In a medium the refractive index varies with wavelength (dispersion), so the speed \(v=c/n\) depends on the wavelength of light.
Q7
In a Young’s double-slit experiment, the slit separation is doubled. To maintain the same fringe spacing on the screen, the screen-to-slit distance D must be changed to :
- A. \(\sqrt{2}\,D\)
- B. \(\frac{D}{2}\)
- C. \(\frac{D}{\sqrt{2}}\)
- D. 2DCorrect
Explanation. Fringe width \(\beta=\frac{\lambda D}{d}\). If d becomes 2d, D must become 2D to keep \(\beta\) unchanged.
Q8
Emission of electrons by the absorption of heat energy is called ________ emission.
- A. field
- B. thermionicCorrect
- C. photoelectric
- D. secondary
Explanation. When a metal is heated, electrons gain enough thermal energy to escape; this is thermionic emission.
Q9
The mass of a \({}^{7}_{3}Li\) nucleus is 0.042 u less than the sum of the masses of all its nucleons. The average binding energy per nucleon of \({}^{7}_{3}Li\) nucleus is nearly :
- A. 23 MeV
- B. 46 MeV
- C. 5.6 MeVCorrect
- D. 3.9 MeV
Explanation. Binding energy \(=0.042\times931\approx39.1\) MeV. Per nucleon \(=39.1/7\approx5.6\) MeV.
Q10
The principle based on which a solar cell operates is :
- A. Photovoltaic actionCorrect
- B. Diffusion
- C. Carrier flow
- D. Recombination
Explanation. A solar cell converts light energy directly into electrical energy by the photovoltaic effect.
Q11
The particle which gives mass to protons and neutrons are :
- A. Nano particle
- B. Higgs particleCorrect
- C. Bulk particle
- D. Einstein particle
Explanation. According to the Standard Model, particles acquire mass through the Higgs field, whose quantum is the Higgs particle (boson).
Q12
The potential energy of a magnetic dipole whose dipole moment is \(\vec{p}_m=(-0.5\hat{i}+0.4\hat{j})\) \(Am^2\) kept in uniform magnetic field \(\vec{B}=2\hat{j}\) T :
- A. 0.1 J
- B. \(-\)0.1 J
- C. 0.8 J
- D. \(-\)0.8 JCorrect
Explanation. \(U=-\vec{p}_m\cdot\vec{B}=-(0.4\times2)=-0.8\) J.
Q13
Which of the following statement is/are true for equipotential surface ?
- A. The potential is different for different equipotential surfaces.
- B. Electric field must always be normal to equipotential surface.
- C. Work done to move a charge between any two points is zero.
- D. All of the aboveCorrect
Explanation. Different equipotential surfaces have different potentials, the field is always perpendicular to them, and no work is done in moving a charge between two points on the same surface. So all statements hold (statement (c) refers to points on the same surface).
Q14
Inside the calcite crystal, along the optic axis, the ratio of velocities of extraordinary ray to ordinary ray is :
- A. 2 : 1
- B. 1 : 1.5
- C. 1 : 2
- D. 1 : 1Correct
Explanation. Along the optic axis there is no double refraction: the ordinary and extraordinary rays travel with the same velocity, so the ratio is 1 : 1.
Q15
The range of the antenna is 64 km, then its height is (Radius of the earth is 6400 km) :
- A. 32 m
- B. 0.32 kmCorrect
- C. 640 m
- D. 32 km
Explanation. \(d=\sqrt{2Rh}\Rightarrow h=\frac{d^2}{2R}=\frac{64^2}{2\times6400}=\frac{4096}{12800}=0.32\) km.
